This is an epic to track improvements to coder agent graceful shutdown.

In no particular order:

• fix(agent): Allow signal propagation when running as PID 1 #6141
• feat(agent): Handle signals and shutdown gracefully #5914
• Research and document how to gracefully shut down agents using different terraform providers (update templates) #6174
• Graceful shutdowns for coder agents and shutdown scripts #6175
• Refactor coder agent SSH server for better process and connection control #6177
• Decide upon and document agent shutdown behavior and guarantees #6179
• Document best practices for safer shutdowns

Related: #4677, #6139